Spin-crossover behavior of neutral iron(iii) complexes with salicylaldehyde thio-, seleno- and semicarbazone ligands: Experiment and theoretical analysis

Article abstract of DOI:10.1039/c9dt01404g

The iron(iii) complex [Fe(Hsemsal)(semsal)]·3H₂O (1) (H₂semsal-salicylaldehyde semicarbazone) has been synthesized and characterized by powder and single crystal X-ray diffraction, and magnetic susceptibility measurements. Crystal st

Full text of DOI:10.1039/c9dt01404g

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Blagov, V. Lazarenko, L. Zorina, A. Vasiliev, V. Krapivin, R. Svetogorov, O. Maximova, S. Simonov and E. B.
Yagubskii, Dalton Trans., 2019, DOI: 10.1039/C9DT01404G.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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ARTICLE
Spin-crossover behavior of neutral iron(III) complexes with
salicylaldehyde thio-, seleno- and semicarbazone ligands:
experiment and theoretical analysis
Received 00th January 20xx,
Accepted 00th January 20xx
a,d
b
c
Nataliya G. Spitsyna, *a Maxim A. Blagov,
Vladimir A. Lazarenko, Leokadiya V. Zorina,
DOI: 10.1039/x0xx00000x
d,e,f
a,b
b
Alexander N. Vasiliev,
Vladimir B. Krapivin, Roman D. Svetogorov, Olga V. Maximova, d,e
Sergey V. Simonov, c Eduard B. Yagubskii a
The iron(III) complex [Fe(Hsemsal)(semsal)]·3H2O (1) (H2semsal - salicylaldehyde semicarbazone) has been synthesized and
characterized by powder and single crystal X-ray diffraction, and magnetic susceptibility measurements. Crystal structure
analysis showed that the complex form neat stacks stabilized by hydrogen-bonding through water molecules and π–π
interactions between phenolate rings of ligands. The complex does not exhibit the spin-crossover phenomena and remains
in the high-spin state down to 2 K. The DFT calculations were performed for a series of neutral Fe(III) complexes, and the
influence of the N2S2O2, N2Se2O2 and N2O4 coordination environment on the spin transition in these complexes was traced.
The effect of substituents in the benzene ring of salicylaldehyde on stabilization of the HS or LS states in complexes of this
type was analyzed.
of most interest.7-11 These ligands are the products of
condensation reaction between salicylaldehydes (sal) and
thiosemicarbazides (th)/selenosemicarbazides (se) (for
Introduction
Compounds exhibiting spin-crossover (SCO) phenomena are
routinely observed in d4–d7 transition metal ions-based
compounds, with iron(II/III) complexes being the most
common.1-3 In case of iron(III) (d5) complexes with specific
ligands, thermally induced reversible SCO is associated with
the switching between low spin (LS, S=1/2) and high spin (HS,
S=5/2) states of iron(III) ion under temperature change.
Noteworthy, the SCO ferric complexes are characterized as
more robust with a wide range of choice of ligands. The ability
to control the switch between two spin states by external
stimuli makes these compounds attractive for applications,
e.g. in displays, sensors, molecular switches and information
storage devices.2-8
example, H2thsal, H2sesal, H2th5Cl-sal и H2th5Br-sal).
Among SCO systems the iron(III) complexes containing two
tridentate O,N,S-coordinating thiosemicarbazonate ligands are
a. Institute of Problems of Chemical Physics, RAS, Chernogolovka, MD 142432,
Russia.
b. National Research Center “Kurchatov institute”, Moscow 123182, Russia.
c. Institute of Solid State Physics, RAS, Chernogolovka, MD 142432, Russia.
d. Lomonosov Moscow State University, Moscow 119991, Russia.
e. National University of Science and Technology "MISIS", Moscow 119991, Russia.
f. National Research South Ural State University, Chelyabinsk 454080, Russia.
Electronic Supplementary Information (ESI) available: Photo of the crystalline
sample of 1 (Figure S1); TGA curve (Figure S2); XRPD patterns (Figure S3), observed
and simulated for 1; experimental and calculated FT-IR spectra for the H2semsal
(Figure S4); selected experimental and calculated IR vibrational modes of H2semsal
(Figure S5); molecular structures of anionic complexes with N2O4 coordination
environment (Figure S6); energy-level diagram of α-spin MOs for Fe(III) neutral
complexes (Figure S7); Tables S1–S5, structural details and comparisons; Tables
Scheme 1. Scheme of formation of the neutral and anionic
complexes of Fe(III) (X= O, S, Se), where R- is a substitute in the
benzene ring of salicylaldehyde. A and B are mono- and di-
anionic forms of ligand, respectively.
Depending on pH of the reaction medium, these ligands
S6–S35 details of experimental methods, and calculations. See can form neutral (I) or anionic (II) iron(III) coordinated
DOI: 10.1039/x0xx00000x
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complexes (Scheme 1), which demonstrate unique properties: (0,43 mmol, 180 mg) in water (5 ml) was addedVidewroAprtiwcleisOenlitnoe
multistep thermally-induced spin transitions (from 2 to 8 the stirred reaction mixture, after then DitOwI: a10s.1s0t3ir9r/eCd9DaTt01r4o0o4mG
steps), LIESST (light-induced excited-spin-state trapping) effect, temperature for 6 hours. The resultant brown precipitate was
anomalous temperature dependence of the real part of the collected by filtration, washed with water. Recrystallization of
dielectric constant, cooperative SCO behavior and a high T1/2 the crude solid from acetone afforded black single crystals 1
value up to 260 K.8,12-23 In addition, thiosemicarbazone and (see Figure S1), which were collected and dried in vacuo over
some semicarbazone complexes with transition metal ions P2O5. Suitable single crystals were chosen for X-ray diffraction.
∙
demonstrate broad chemotherapeutical and biological Yield 65 %. Calcd. (found%) for [Fe(Hsemsal)(semsal)] 3H2O,
activities.18,19
(Mw = 465.22 g/mol) C16H21FeN6O7: C, 41.31 (41.30); H, 4.52
In contrast to the Fe(III) complexes with H2thsal (N2S2O2) (4.33); N,18.06 (17.98); Fe, 12.04 (11.66) %. The compound 1 is
ligand family, the structure and magnetic properties of Fe(III) stable in air, and it was dried in vacuo for several hours prior to
ones with salicylaldehyde semicarbozone ligand (H2semsal, all characterization. The phase purity of 1 was confirmed by
N2O4 coordination) have not been substantially studied yet. comparison of the simulated and experimental powder
Several attempts to obtain the single crystals of Fe(III) diffraction patterns (Figure S3).
complexes with H2semsal ligand suitable for complete X-ray
diffraction analysis appeared to be unsuccessful.11,19
Thermogravimetry and differential scanning calorimetry
Herein we report synthesis and characterization of the
iron(III) neutral complex [Fe(Hsemsal)(semsal)]·3H2O (1) The thermal stability of solid sample were investigated by
(H2semsal salicylaldehyde semicarbazone). The DFT using a NETZSCH (Germany) STA 409 C Luxx thermal analyzer,
-
calculations were performed and the effects of ligand which allows simultaneous thermogravimetry (TG) and
environment on the spin transition in the iron(III) complexes differential scanning calorimetry (DSC) measurements.
with N2S2O2, N2Se2O2 and N2O4 donor sets (Scheme 1) were
traced. The influence of substituents in the benzene ring of IR spectroscopy
salicylaldehyde on stabilization of the HS or LS state in these
complexes was analyzed.
The ATR FT-IR absorption spectra were recorded at a room
temperature in the range 4000–400 cm-1 (50 scans, resolution
4 cm-1) using FT-IR Bruker model Vertex 70V spectrometer
(Germany). The ATR measurements were performed using
Experimental section
Methods and Materials
Platinum-ATR (Bruker) accessory equipped with
diamond crystal. The samples were measured as solids.
a pure
Materials were purchased from commercial sources and were
used without further purification. For syntheses the Sigma- Single-crystal structure determinations
∙
Aldrich Fe(NO3)3 9H2O reagent grade was used.
The purity of H2semsal and complex 1 was verified by X-ray diffraction data were collected at the X-ray Structural
determining the carbon, hydrogen, and nitrogen content with Analysis (XSA) beamline (=0.80246 Å) of Kurchatov
a Vario MICRO Cube Elementar GmbH Analysis Service. The Synchrotron Radiation Source. All data were collected at 100 K
samples were prepared in tin boats. All samples were and in direct (=0) geometry. In total, ~400 frames were
measured at least twice, and the average was used.
collected with oscillation range of 1 in  scanning mode.
Absorption correction was applied by using program Scala.25
The data were indexed and integrated by using of
iMOSFLM from CCP4 software package.26 Structure was solved
Synthesis
The salicylaldehyde semicarbazone (H2semsal) ligand and by direct methods (intrinsic phasing) in SHELXT27 and refined in
[Fe(Hsemsal)(semsal)]·3H2O (1) were prepared according to Olex2 software28 by a full-matrix least-squares method on F2
improved procedure described earlier.11,19,24
with anisotropic displacement, hydrogens were placed
Synthesis of H2semsal. A mixture of equimolar quantities according to geometry and residual electron density peaks.
of salicylaldehyde (16.5 mmol, 2.0 g) with semicarbazide Crystal data for 1: C16H21FeN6O7, Mw = 465.24 g/mol,
hydrochloride (16.5 mmol, 1.6 g) in methanol (75 ml) was monoclinic, space group P21/c (no. 14), a = 17.587(4) Å, b =
refluxing for 16 hours to give colorless solution and then slowly 12.556(3) Å, c = 9.0534(18) Å, β = 93.77(3)°, V = 1994.8(7) Å3, Z
cooled to room temperature. The H2semsal compound as = 4, T = 100(1) K, crystal size 0.05 × 0.12 × 0.12 mm3,
white crystalline precipitate was separated by filtration, synchrotron radiation, λ = 0.80246 Å, μ = 1.128 mm-1, Dcalc
=
washed with methanol and dried in vacuo over P2O5. Yield 73 1.549 g/cm3, 21114 reflections measured (4.504° ≤ 2Θ ≤
%. Calcd. (found%) for H2semsal (Mw = 179.18 g/mol), 61.706°), 4319 unique (Rint = 0.0668, Rsigma = 0.0520) which
C8H9N3O2: C, 53.63 (54.01); H, 5.03 (5.04); N, 23.46 (23.52) %. were used in all calculations. The final R1 was 0.0603 (for 3812
∙
Synthesis of [Fe(Hsemsal)(semsal)] 3H2O (1). The water reflections with I > 2σ(I) and 305 parameters refined) and wR2
(15 ml) solution of H2semsal (0.86 mmol, 150 mg) and KOH was 0.1515 (all data), GooF = 1.034. Crystallographic data for 1
(1.72 mmol, 97 mg) was stirring at room temperature for 30 have been deposited with the Cambridge Crystallographic Data
∙
min, yielding a yellow solution. Solution of Fe(NO3)3 9H2O Center, CCDC 1903068. The supplementary crystallographic
2 | J. Name., 2012, 00, 1-3
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data can be obtained free of charge from the Cambridge Results and Discussion
Crystallographic Data Centre via
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DOI: 10.1039/C9DT01404G
http://www.ccdc.cam.ac.uk/data_reque st/cif.
Thermal Properties
X-Ray powder diffraction
The thermogravimetric analysis of 1 revealed a mass loss of
9.11% in the temperature range 75-200°C, which corresponds
X-ray powder diffraction (XRPD) study of the samples was to a loss of ~3 molecules of lattice H2O (1·3H2O, calcd 9.98%),
conducted at the XSA beamline of Kurchatov Synchrotron Figure S2. The next weight-loss step appears at a temperature
Radiation Source with the use of Belok endstation.29 The above 200 °C with an endothermic DSC peak at 280 °C, which
monochromatic synchrotron beam with the wavelength of corresponds to the decomposition of the complex.
0.788 Å (photon energy 15734 eV) was used. Diffraction
patterns were collected in a transmission mode with 400 μm IR spectra
beam and the distance between the sample and 2D Rayonix
SX165 detector was 120 mm. The sample was placed in a The comparison IR spectra of the ligand (H2semsal) and
∙
cryoloop of 300 μm in size and rotated around the horizontal complex [Fe(Hsemsal)(semsal)] 3H2O (Figure 1) indicate that
axis during the measurement, which made it possible to the donor atoms (oxygen from the phenolic hydroxyl and C=O
average the diffraction patterns according to the orientations group; nitrogen from azomethine group) are engaged in
of the sample. The exposure time was 3-5 minutes. To coordination (Scheme 1). The strong bands at 3465 cm-1 and
calibrate the sample-detector distance we need
polycrystalline standard with known position of the valence vibrations of the phenolic hydroxyl group (Figures S4
diffraction peaks; in this series of measurements, Na2Ca3Al2F14 and S5),39 evolve in the IR spectrum of complex 1 as following:
(NAC NIST SRM) powder was used. the first one almost disappears and the second shifts toward
a
1265 cm-1 in the IR spectrum of ligand, which are assigned to
a
The two-dimensional diffraction patterns obtained on the higher energies 1291 cm-1, indicating the involvement of
detector were further integrated to the standard form of the oxygen in the coordination of iron ion. One of the strongest
dependence of the intensity on the scattering angle I (2θ) bands of the azomethine group ν(C=N) located at 1586 cm-1 in
using the Fit2D program and Dionis software.30 The adequacy the spectrum of ligand shifts about 10 cm-1 toward higher
of unit cell and space group was confirmed by the Rietveld energies in the case of complex 1.
method using Jana2006 software.31
Magnetic Measurements
Magnetic properties of [Fe(Hsemsal)(semsal)]·3H2O were
investigated using the VSM option of “Quantum Design”
Physical Properties Measurements System PPMS ‒ 9T on the
fine powder sample with mass of about 5 mg placed into the
plastic capsule. The temperature dependence of dc magnetic
susceptibility  of the title compound has been taken at
warming in a magnetic field B=0.1 T.
Computation Details
All calculations were carried out using the Gaussian 09
computational package32 with B3LYP*33 (15% Hartree-Fock
exchange) hybrid functional, specially developed by Reiher and
co-workers on the base of B3LYP,34,35 to get more accurate
results of the energy splitting between states with different
spin multiplicity.36 Calculations of the iron(III) complexes were
performed with an unrestricted wave functions. Molecular
geometries and orbitals (MOs), electronic energies (Eel) and
vibrational frequencies were calculated using the 6-
311++G(d,p) basis set.37 Zero point energies (EZPV) for the total
energy E0 (E0 = Eel + EZPV) were obtained by the harmonic
oscillator approximation. Natural atomic charges were
obtained using the NBO 3.1 module38 in Gaussian 09. The
atomic coordinates for starting geometry of neutral complexes
were taken from the single crystal structural data (Scheme
1).20, this work
Figure 1. Experimental ATR FT-IR absorption spectra for the
∙
H2semsal (red line) and [Fe(Hsemsal)(semsal)] 3H2O (black
line), T = 293 K, without preliminary sample preparation.
Calculated (B3LYP*/6-311++G(d,p)) IR vibrational frequencies
for the [Fe(Hsemsal)(semsal)] complex in HS state (blue bars).
A very strong band ν(C=O) at 1679 cm-1 in the ligand
spectrum splits into two bands at 1676 cm-1 (Hsemsal-) and
1649 cm-1 (semsal2-) in the IR spectrum of 1. Compared to the
spectrum of free ligand, a wide band with a complex structure
related to the ν(NH2) modes is slightly shifted towards higher
frequencies in 1 (3400–3000 cm-1), Figure 1. Analysis of the IR
spectra showed that the ligand chelates the ferric ion in the
tridentate manner, as in a number of related anionic
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complexes of Fe(III).20 The complex 1 contains three molecules monoanionic keto form with proton at the hydrazinic nitrogen
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DOI: 10.1039/C9DT01404G
of crystallization water (see structural description). This atom N(2) and deprotonated dianionic enol form (Scheme 1).
correlates with the presence of broad and rather intense The dianionic ligand forms shorter coordination bonds to the
absorption bands in the spectral region of 3500–3400 cm-1 central metal ion in comparison with the monoanionic one
related to the ν(OH) modes of water (inset in Figure 1).
(Table 1, maximal difference is 0.07 Å for Fe-Ocarbonyl) whereas
all covalent bonds in the semicarbazone moieties are
practically identical in both tautomers (±0.01Å, Table S1). The
dianionic semsal ligand is more planar in comparison with
Crystal structure
Complex 1 crystallizes in the monoclinic P21/c space group. monoanionic Hsemsal unit (Figure 3). The dihedral angle
The asymmetric unit includes one [FeIII(Hsemsal)(semsal)] between OCN3 and OC7 parts of the ligand is 18.02(9)ᵒ for
fragment and three solvent water molecules, all are in general Hsemsal and only 2.08(7)ᵒ for semsal. The angle between the
positions.
average Hsemsal and semsal planes in 1 is equal to 84.61(2)ᵒ.
Figure 3. Different degree of planarity of Hsemsal and semsal
ligands in the complex 1.
Figure 2. Asymmetric unit in 1 with atom numbering scheme
(ORTEP drawing with 50% probability ellipsoids).
An ORTEP drawing of 1 is shown in Figure 2, key bond
distances and angles are listed in Table 1.
Table 1. Selected bond lengths (Å) and angles (°) in
[FeIII(Hsemsal)(semsal)].
Fe1 O2 1.937(2)
Fe1 O4 2.109(2)
Fe1 N1 2.134(3)
Fe1 O1 1.888(3)
Fe1 O3 2.030(2)
Fe1 N4 2.110(3)
O2 Fe1 O4 154.63(9)
O2 Fe1 N1 84.12(9)
O4 Fe1 N1 73.94(9)
O2 Fe1 N4 111.55(9)
O4 Fe1 N4 89.86(9)
O2 Fe1 O3 86.46(10)
O1 Fe1 O2 96.29(11)
N1 Fe1 N4 163.79(10)
O1 Fe1 O3 161.41(9)
O1 Fe1 N4 86.60(10)
O3 Fe1 N4 75.35(9)
O1 Fe1 N1 96.04(10)
O3 Fe1 N1 102.53(9)
O1 Fe1 O4 98.51(10)
O3 Fe1 O4 86.04(9)
The Fe(III) ion in the metal complex has a distorted
octahedral environment of two nitrogen and four oxygen
atoms of two chelating salicylaldehyde semicarbazone ligands.
The chelating agents are in two tautomeric forms:
Figure 4 Projection of the structure 1 along c. The hydrogen
bonding through six H2O molecules near (1/2, 1/2, 0) center
are shown by red dotted lines.
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The bond lengths analysis reveals that the complex 1 is in a stacks: stack 1 is composed of Hsemsal units, stack 2 consists
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high spin state at 100 K. The average bond lengths Fe-N, Fe- of semsal units (Figure 5). The stacks areDsOliIg:h10tl.1y0d39if/fCe9rDeTn0t1f4r0o4mG
Ophenolate and Fe-Ocarbonyl are 2.123, 1.912 and 2.070 Å, each other. The interplane angle is 12.22(7)ᵒ and 40.02(6)ᵒ in
respectively. Previously only one crystallographically the stack 1 (Figure 5b) and 2 (Figure 5c), respectively. The
characterized phenolic semicarbazone complex of Fe(III) has average distances between the planes are 3.04 Å in 1 and 3.51
been published.18 This ferric adamantyl semicarbazone (asc) Å in 2. The stack 1 with less interplane angle has larger amount
complex of the [Fe(asc)2]ClO4·0.5EtOH composition is in HS of shortened C…C contacts than the stack 2 (the contacts < 3.6
state and fully protonated at the hydrazinic nitrogens. The Fe- Å are shown by dotted lines in Figure 5a).
N (2.109(3) Å) and Fe-O average bond lengths (phenolate
1.902(2) Å and carbonyl 2.090(4) Å) are very similar to the Table 2. Hydrogen bonds geometry in 1.
corresponding values in 1. The HS complexes of Fe(III) are also
characterized by noticeable distortion of the coordination
octahedron: the O-Fe-O and N-Fe-N angles in 1 differ from
180ᵒ (Table 1). In the related low-spin complexes of Fe(III) with
hydrazone or thiosemicarbazone ligands the central
octahedron has much less distortion and the lengths of Fe-
ligand bonds are shorter by 0.1-0.3 Å.20,21
D
H
A
d(D-H)/Å d(H-A)/Å d(D-A)/Å D-H-A/°
N2 H2 O5 0.80(4) 1.91(4) 2.708(4) 178(4)
N3 H3B O71 0.84(5) 2.05(5) 2.871(4) 168(4)
N6 H6B O72 0.90(3) 2.09(3) 2.958(4) 162(4)
O5 H5A O63 1.03(5) 1.73(5) 2.748(3) 172(4)
O5 H5B O24 0.93(5) 1.92(5) 2.823(4) 163(4)
O6 H6C N5 1.01(5) 1.74(5) 2.725(3) 164(4)
O6 H6D O42 0.90(5) 1.94(5) 2.790(3) 158(4)
O7 H7A O6 0.95(5) 1.83(5) 2.749(4) 162(4)
O7 H7B O35 0.97(5) 1.80(5) 2.768(3) 174(4)
11-x,1-y,1-z; 2x,0.5-y,0.5+z; 3x,1+y,+z; 4x, 1.5-y, z-0.5; 51-x, y-0.5,
1.5-z
Water molecules (they are marked by thick black bonds in
Figure 4) are located in the cavities around (1/2, 1/2, 0) and
(1/2, 0, 1/2) centers. They are involved in strong hydrogen
bonding both with each other and with surrounding Fe
complexes which locks the layers in pairs. In these interactions
each of three independent water molecules plays mixed
donor-acceptor role in the O-H…O, O-H…N and N-H…O
contacts. The hydrogen bonds geometry is described in Table
2. Note that the direct hydrogen bonding between the Fe
complexes without water mediation is not observed.
Magnetic Properties
The temperature dependence of dc magnetic susceptibility 
of the title compound 1 taken at warming in a magnetic field B
= 0.1 T is shown in Figure 6. In the wide temperature range,
the (T) curve follows the Curie-Weiss law with inclusion of the
temperature independent term  = 0 + C/(T ‒ ). The fitting
of experimental data in the range T = 200 – 300 K results in 0
= ‒ 1.1×10-3 emu/mol. This term arises from the combination
of diamagnetic Pascal constants of individual atoms in the title
compound40 and weak diamagnetic signal of the sample
holder. The Curie constant C = 4.57 emuK/mol corresponds to
the expected value for the Fe(III) ions in the high spin state S =
5/2. Negative value of the Weiss temperature  = ‒ 2.6 K
evidences weak antiferromagnetic interaction between
magnetic centers in 1. The presence of antiferromagnetic
interaction between magnetic centers in 1 could occur due to
the presence of π-staking interactions of the ligands of
adjacent molecules in the layer (Figures 4 and 5). The
Figure 5 The bc layer (a), stack 1 (b) and stack 2 (c). Intrastack
C…C contacts < 3.6 Å are shown by dotted lines.
The [Fe(Hsemsal)(semsal)] units are packed into 2D-layers
parallel to the bc plane in such a way that in between of two
adjacent layers either –NH2 or phenyl ends of chelating agents
are located (Figure 4). Adjacent molecules in the layer interact
by π-stacking of the ligands. There are two independent π-
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extrapolation of the fitting curve to low temperatures is shown %), while the increase of Fe-O(A, B) bond length is less
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by dashed line in the main panel of Figure 6.
pronounced (1.30-4.17 %) (Table S2D)O. I:T10h.i1s039t/eCn9dDeTn0c14y04iGs
The inset to Figure
6
represents the temperature conformed experimentally for neutral complex [Fe(Hth5Br-
21
∙
dependence of *T product where * =  ‒ 0 is the magnetic sal)(th5Br-sal)] H2O (Table S3).
susceptibility corrected for temperature independent
diamagnetic contribution. Down to ~ 50 K, the value of this
product is almost temperature independent reflecting the
constant value of effective magnetic moment eff = (8C)1/2B.
Below this temperature, the *T product diminishes reflecting
presence of weak antiferromagnetic interaction between
magnetically active centers. This trend can be reproduced
without considering a zero-field splitting of the iron ions.41
Figure 7. Dependence of the calculated (B3LYP*/6-311 ++G
(d,p)) difference of total energies between HS and LS states for
the neutral complexes: (S, ●), (Se, ▲) and (O, ■) with different
substituents (R) on Hammett parameter (σp). The dashed line
marks the boundary of the SCO possible area: ΔE0 (HS-LS)>0
(SCO possible), ΔE0 (HS-LS)<0 (SCO impossible, stable HS state).
The MO energies are lowering due to elongation of the Fe-
Figure 6. The temperature dependence of dc magnetic N(A, B) and Fe-X(A, B) bonds induced by the geometry changes
susceptibility  in 1 taken at B = 0.1 T. The dashed line is the from the doublet (LS) spin state to the sextet (HS) spin state,
fitting by the Curie-Weiss law. The inset represents the since the eg-like orbitals form antibonding overlaps with the
temperature dependence of the product *T, where * =  ‒ N(A, B) and X(A, B) lone pair orbitals. As a result of these
0.
geometry changes, the sextet spin state becomes more
energetically stable.42
Nevertheless, there are some works that reported spin
Since a thermally induced SCO is an entropy driven
transition in the ferric complexes with N2O4 coordinated process, the major contribution comes from changes in the
-
-
′
→
environment ([Fe(azp)2] , H2azp=2,2 -azodiphenol; [Fe(aznp)2] intramolecular vibrations (Svib) and spin multiplicity LS HS (Sel
2
′
~
H2aznp=(2 -hydroxyphenylazo)-2-hydroxynaphthalene) (Figure
S6).17 That is unusual, because an oxygen atom belongs to SCO is the energy proximity of the LS to HS state. The results of
25%). Therefore, the main criterion for the appearance of
weak field ligands. Spin transition in these complexes is DFT calculations of total energy difference between HS and LS
―
achieved by σ-bonding of nitrogen atom of azo groups ( N=N states (ΔE0 (HS-LS)) for the iron(III) neutral complexes of R-
―
) to iron d-orbitals, that increases a ligand-field splitting salicylaldehyde thio-, seleno-, semicarbazone are shown on
energy.16,17
the Figure 7 and Table S4. According to DFT calculations, the
neutral complexes with S2N2O2 and Se2N2O2 coordination may
experience the thermally induced spin transition, while N2O4
coordination environment implies the stabilization of HS state
DFT calculations
To determine the influence of coordination X2N2O2 X=O; S; Se of iron ion. All dependences demonstrate similar positive
(A and B, Scheme 1) environment of Fe(III) neutral complexes linear relationship between ΔE0 (HS-LS) and “R” substituent
on the spin-crossover the DFT calculations were performed. Hammett parameter (σp).43 This also implies that
The optimized coordination structures for neutral complexes iron/phenoxide π-bonding is important for these compounds.
agree quite well with the corresponding crystal structures. Electron-withdrawing substituents (5Cl-, 5NO2-) would
However, experimental bond lengths of Fe-N(A) and Fe-X(А) strengthen the ligand field by reducing ligand–metal π-
for neutral complexes (R=H) are somewhat different from donation and stabilize the low-spin states of the complexes.
calculated values. These bond distances might be considerably Electron-donating substituents (5MeO-) on the contrary would
influenced by the crystal packing.42 The calculated elongation weaken the ligand field by enhancing ligand–metal π-donation
of the bond lengths in the neutral complexes upon the LS HS and stabilize the high-spin states of the complexes.44 Thus, the
→
transition are following: Fe-N(A) (14.63-13.89 %), Fe-X(А) regulation of spin transition parameter (T1/2) for these
(10.70-8.08 %), Fe-N(B) (11.13-10.85 %) and Fe-X(В) (6.16-4.94
6 | J. Name., 2012, 00, 1-3
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Figure 8. HOMO, HOMO-1 and HOMO-2 (left); HOMO, HOMO-1 (right) surfaces calculated (B3LYP*/6-311++G(d,p)) for (A) (left)
and (B) (right) ligand forms, based on the optimized structures coordinates of the Fe(III) neutral complexes (I) (X= O, S, Se; R=H)
in the HS state. A contour of 0.04 eÅ–3 has been used for the MOs plots.
compounds can be realized by varying the substituents in
benzene ring of salicylaldehyde It`s important to mention that
the linear dependence (Figure 7) reflects only electronic
influence of ligand substituents. Similar linear dependence was
obtained for spin-crossover saltrien-type Fe(III) complexes,
which contain analogous fragment of R-salicylaldehyde in their
structure.44 The observed correlation may vary under the
influence of the crystal packing features.20
from each optimized the Fe(III) neutral complexes in HS state.
The N σ-like orbitals of ligand were observed in HOMO-1,
HOMO-2 for A form and HOMO-1 for B form (Figure 8).
Here we conclude that σ-bonding of nitrogen atoms of
―
imino ( N(im)=) groups have a little influence on the increase
of iron orbitals splitting energy, what provides the
d
stabilization of HS state for neutral complexes with X=O (weak
field ligand). The same example of the stabilization of HS state
is observed for [Fe(phsal)2]- anionic complex (H2phsal= N-(2-
hydroxyphenyl)salicylaldimine)) with N(im)2O4 coordinated
environment. However [Fe(azp)2]- and [Fe(aznp)2]- complexes
Magnetic
measurement,
carried
out
for
∙
[Fe(Hsemsal)(semsal)] 3H2O complex, confirms results of DFT
calculations. Noteworthy, the value of ΔE0 (HS-LS) for the
neutral complexes where X=S slightly exceeds for the neutral
complexes where X=Se. The larger value of ΔE0 (HS-LS)
indicates the shift of T1/2 value towards higher temperatures.
The similar experimental observation for Fe(III) anionic
complexes was reported in previous works.11
―
―
(N(azo)2O4 coordination) with azo
(
N=N
)
group
demonstrate SCO transition since σ-bounding of this groups
with iron d orbitals leads to the enhancement of the ligand-
field splitting energy (Figure S6).16 The HOMO-1 σ-type
coefficients around the imino group in the B ligand form were
much larger than in the A ligand form. Besides, these
coefficients for the B ligand with X=S were slightly larger than
in the case of X=Se while these coefficients for X=S and X=O
are quite equal (Figure 8). Therefore, larger σ-type coefficients
might result into the enhancement of the ligand-field splitting
energy. This conclusion correlates with the calculated values of
ΔE0 (HS-LS) (Table S4). Moreover, iron e*g-like orbitals for the
complex with X=S are more destabilized comparing to X=Se,
since thsa2- ligand has lager σ-type coefficients around the
imino group (Figure S7).
The energy-level diagrams of the occupied α-spin
molecular orbitals for the HS Fe(III) neutral complexes (X=S, Se,
O; R=H) are shown on the Figure S7. The observed distribution
of the iron eg-like and t2g-like orbitals in MOs corresponds to
calculated ΔE0 (HS-LS) values (Table S4). However, the mixed
iron d orbitals complicate detailed MO analysis for the
complexes. The mixing of the iron d orbitals leads to the
increase of the degree of the overlap between metal orbitals
and σ-/π- donor ligand orbitals.45
The considered ligands include π-donor atoms: X= O, S and
Se. Oxygen has higher electronegativity than sulfur and
selenium. Therefore it has low-lying energy t2g-orbitals what
reduces iron d orbitals splitting energy. This fact correlates
with the energy-level diagrams of MOs: iron t*2g -like orbitals
of neutral complex with X=O are more destabilized compared
to X=S and Se (Figure S7). The analysis of natural atomic
charges on Fe for these complexes indicates that the degree of
π-bonding in the complex with X=Se is stronger than in
complex with X=S.46 This corresponds to the smaller natural
To corroborate the results described above we performed
analogous DFT calculations for the anionic forms of complexes
(X= O, S, Se) with R=H, where Fe(III) is coordinated by two B
ligands. Computation details are produced in the supporting
information (Table S5). The ΔE0 (HS-LS) values for anionic
→
complexes increase in row: O(-11.05 kJ/mol) Se(9.80 kJ/mol)
→
S(12.46 kJ/mol). Compared to the neutral complexes, the
ΔE0 (HS-LS) values for anionic complexes increase by 8.24
kJ/mol for X=S, 7.62 kJ/mol for X=O, and 6.86 kJ/mol for X=Se
(Table S4). Thereby, the B form ligand gives much larger value
of the ligand-field splitting energy because of the nitrogen
atom of imino group has larger σ-type coefficients. Comparing
to X=Se, B form ligand with X=S may also give lager values of
ΔE0 (HS-LS) due to larger σ-type coefficients of imino group
(Figure 7). However, the higher value of ΔE0 (HS-LS) = -11.05
>
atomic charge on Fe(HS): 0.781 e (X=S)
0.726 e (X=Se). A
higher degree of π-bonding may also be the reason of value E0
(HS-LS) decreasing for the complex with X=Se. The observed
energy discrepancy of the distribution of MOs containing iron
t*2g-like orbitals for S and Se is the result of strong mixing of d
orbitals.
The single-point calculations for the A and B ligand forms
were also performed using the atomic coordinates clipped
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kJ/mol for anionic complex with X=O does not provide the the framework of Increase Competitiveness ProgVriaewmAroticfleNOUnlSinTe
possibility of SCO transition.
“MISiS” Grant No. K2-2017-084; by Act 2D1O1I:o1f0t.1h0e39G/Co9vDeTr0n1m40e4nGt
of Russian Federation, Contracts No. 02.A03.21.0004 and No.
02.A03.21.0011, and Russian Foundation for Basic Research
Grant No.18-32-00153.
Conclusions
∙
Neutral iron(III) complex [Fe(Hsemsal)(semsal)] 3H2O (1) with
N2O4 coordination environment was synthesized. Crystal
structure analysis showed that the Fe(III) ion in 1 has a
distorted octahedral coordination of two nitrogen and four
oxygen atoms owned by two tautomeric forms of the
salicylaldehyde semicarbazone ligand: Hsemsal- (keto) and
semsal2- (enol).
Notes and references
1
2
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The magnetization measurements confirmed the
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down to liquid helium temperatures. Negative value of the
Weiss temperature indicates the presence of weak
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The DFT calculations were performed for the iron(III)
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Partially the study was carried out within the state assignment
for the ISSP RAS. This work was supported partly by the
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A novel neutral complex [FeIII(Hsemsal)(semsal)]·3H2O was synthesized and its magnetic properties, crystal and electronic
structure were studied.
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